Append the zone ID after compressing the address in IPAddr#to_s

[afurm]

IPAddr#to_s runs compression on the string with the zone already appended, so IPAddr.new("::%eth0").to_s gives "::0%eth0", an IPv4-mapped address with a zone loses its dotted form, and zone ids like %0ab become %ab (a different interface). The fix compresses the bare address, then appends the zone. Test added.

[taketo1113]

Thanks.
I confirmed the following behavior changes.

Zone IDs like %0ab

# Before fix
IPAddr.new("fe80::1%0ab").to_s
#=> "fe80::1%ab"

IPAddr.new("fe80::1%0ab").to_string
#=> "fe80:0000:0000:0000:0000:0000:0000:0001%0ab"

# After fix
IPAddr.new("fe80::1%0ab").to_s
#=> "fe80::1%0ab"

IPAddr.new("fe80::1%0ab").to_string
#=> "fe80:0000:0000:0000:0000:0000:0000:0001%0ab"

"::0%eth0" and IPv4-mapped addresses with a zone ID

# Before fix
IPAddr.new("::0%eth0").to_s
#=> "::0%eth0"

IPAddr.new("::ffff:192.168.1.2%eth0").to_s
#=> "::ffff:c0a8:102%eth0"

IPAddr.new("::ffff:192.168.1.2%eth0").to_string
#=> "0000:0000:0000:0000:0000:ffff:c0a8:0102%eth0"

# After fix
IPAddr.new("::0%eth0").to_s
#=> "::%eth0"

IPAddr.new("::ffff:192.168.1.2%eth0").to_s
#=> "::ffff:192.168.1.2%eth0"

IPAddr.new("::ffff:192.168.1.2%eth0").to_string
#=> "0000:0000:0000:0000:0000:ffff:c0a8:0102%eth0"

The change to the to_s representation of IPv4-mapped IPv6 addresses is also consistent with RFC 5952, which specifies their recommended text representation:

The text representation method noted in Section 4 should be applied
for the leading hexadecimal part (i.e., ::ffff:192.0.2.1 instead of
0:0:0:0:0:ffff:192.0.2.1).

• https://www.rfc-editor.org/info/rfc5952/#section-5